[1][2] More specifically, it determines the amount of conjugated bilirubin in the blood.
Principle: bilirubin reacts with diazotised sulphanilic acid to produce purple coloured azobilirubin.
This was pioneered by the Dutch physician, Abraham Albert Hijmans van den Bergh (1869–1943) of Utrecht.
If a red colour develops immediately it is called a direct positive.
In an indirect positive test, the patient's serum is first treated with alcohol and later mixed with diazo reagent.